In the field of hard disk drives, so-called spiral servo is widely known. In the spiral servo, multi-spiral patterns are well known as intermediate patterns for self-servo writing on a surface of a magnetic disk. Each of the multi-spiral patterns extends from the innermost circumference to the outermost circumference of a recording area along a spiral line. Such a spiral line has a constant inclination of a specified angle with respect to the circumferential lines across the entire recording area.
In a hard disk drive, a head (electromagnetic transducer) reads magnetic information from the multi-spiral patterns as the magnetic disk is rotated. The head is then positioned in the radius direction of the magnetic disk based on the read magnetic information. The head thus positioned is used in writing servo patterns in servo sectors on the magnetic disk.
A multi-spiral pattern comprises a high frequency area. In the high frequency area, magnetic poles are arranged in an alternating manner along the circumference direction. When the electromagnetic transducer traverses across the high frequency area, a high frequency reproduction signal is output. Sync marks are also formed in the multi-spiral pattern along the circumference direction at a predetermined interval. Each of the sync marks forms a gap between high frequency reproduction signals. The interval between such gaps corresponds to a track width. The sync marks function to position the head for each recording track.
In the spiral servo, the head is positioned based solely on a very small displacement decoded when the head traverses across the multi-spiral patterns. Therefore, before starting writing servo patterns between the spiral patterns, another servo pattern (normal servo pattern that is not the multi-spiral patterns, in other words, an auxiliary servo pattern) must be established between the servo patterns within a limited area on the magnetic disk.
In other words, conventionally, to position the head at a position for starting self-servo write (SSW), seed patterns (auxiliary servo patterns) formed on a part of the magnetic disk are used to position the head at the position for starting the write. A detector of the servo information then decodes the timing of decoding gates to follow a multi-spiral reproduction waveform using the similarity between the repetitive run-outs (RROs) of the auxiliary servo patterns and the multi-spiral patterns. In this manner, by changing the use of the auxiliary servo patterns to the use of the multi-spiral patterns, the head is appropriately positioned (also referred to as on-track) based on the multi-spiral patterns.
However, in the conventional technique, because the servo process must be executed for both of the auxiliary servo patterns and the multi-spiral patterns simultaneously within one sampling cycle, the CPU load and the memory capacity of the servo controller increase.
Furthermore, the area for writing final patterns is reduced by the area of the auxiliary servo patterns, and this issue is not ignorable especially in view of the recent reduction in sampling cycles.